Variations to Mendel’s Variations to Mendel’s LawsLaws

Extensions and ExceptionsExtensions and Exceptions

Alterations to Mendel’s RatiosAlterations to Mendel’s Ratios

In these cases, genotypic ratio is as Mendel In these cases, genotypic ratio is as Mendel predicted but phenotypic ratio is alteredpredicted but phenotypic ratio is altered Lethal allele combinationsLethal allele combinations Multiple allelesMultiple alleles Different dominance relationshipsDifferent dominance relationships EpistasisEpistasis Penetrance and expressivityPenetrance and expressivity PleiotropyPleiotropy Genetic heterogeneityGenetic heterogeneity PhenocopiesPhenocopies

Lethal Allele CombinationsLethal Allele Combinations

Multiple AllelesMultiple Alleles

Give a range of phenotypesGive a range of phenotypes Each of us has 2 alleles for any given Each of us has 2 alleles for any given

genegene 1 from mom and 1 from dad1 from mom and 1 from dad

There can be many different alleles for a There can be many different alleles for a genegene Different alleles are formed by mutationsDifferent alleles are formed by mutations

Example of Multiple Alleles -PKUExample of Multiple Alleles -PKU

Phenylketonuria (PKU)Phenylketonuria (PKU) Enzyme that breaks down phenylalanine is deficientEnzyme that breaks down phenylalanine is deficient

• Phenylalanine accumulatesPhenylalanine accumulates There are hundreds of possible allelesThere are hundreds of possible alleles Allelic combinations give rise to different phenotypesAllelic combinations give rise to different phenotypes

• Severe mental retardationSevere mental retardation• Moderate PKUModerate PKU• Mild PKUMild PKU• Asymptomatic PKUAsymptomatic PKU

Different Dominance Relationship –Different Dominance Relationship –Incomplete dominanceIncomplete dominance

The heterozygous The heterozygous phenotype is phenotype is intermediate between intermediate between the homozygous the homozygous dominant and the dominant and the homozygous homozygous recessiverecessive

Blended phenotypeBlended phenotype

Different Dominance Relationship –Different Dominance Relationship –codominancecodominance

Phenotypes of both Phenotypes of both alleles are expressedalleles are expressed

Codominance –Blood TypeCodominance –Blood Type

Codominance –Blood TypeCodominance –Blood Type

EpistasisEpistasis

One gene affects the expression of One gene affects the expression of anotheranother Example: If a dog has the hairless gene, the Example: If a dog has the hairless gene, the

genes that affect hair color will not be genes that affect hair color will not be expressedexpressed

Penetrance and ExpressivityPenetrance and Expressivity Describe degrees of inheritanceDescribe degrees of inheritance Due to multifactorial inheritanceDue to multifactorial inheritance

Expression of a gene is influenced by other genes Expression of a gene is influenced by other genes and by environmentand by environment

Penetrance =all-or-none expression of a genePenetrance =all-or-none expression of a gene 100% of the people who inherit mutant amyloid 100% of the people who inherit mutant amyloid

precursor protein (mAPP) develop Alzheimer disease precursor protein (mAPP) develop Alzheimer disease so mAPP is 100% penetrantso mAPP is 100% penetrant

Expressivity =severity of gene expressionExpressivity =severity of gene expression Polydactyly has variable expressivityPolydactyly has variable expressivity

• Some have an extra toe and others have an extra toe and an Some have an extra toe and others have an extra toe and an extra finger.extra finger.

PleiotropyPleiotropy

Gene affects several functionsGene affects several functions Phenotype is variedPhenotype is varied

Example: Porphyria variegataExample: Porphyria variegata

Genetic HeterogeneityGenetic Heterogeneity

Different genes produce the same phenotypeDifferent genes produce the same phenotype Example:Example:

Hearing loss may be due to one of 132 different Hearing loss may be due to one of 132 different genes that follow autosomal recessive inheritancegenes that follow autosomal recessive inheritance

B, b =gene for hearing loss type 1B, b =gene for hearing loss type 1R, r =gene for hearing loss type 2R, r =gene for hearing loss type 2

BBrr bbRR

BbRr

deaf deaf

NOT deaf

PhenocopyPhenocopy

An environmentally caused trait that An environmentally caused trait that appears to be inheritedappears to be inherited

Example: Phocomelia is a rare genetic Example: Phocomelia is a rare genetic disorder whose effects are mimicked by disorder whose effects are mimicked by the teratogen, thalidomidethe teratogen, thalidomide

Example: AIDs transmission from mother Example: AIDs transmission from mother to offspringto offspring

Mitochondrial GenesMitochondrial Genes

Mitochondrial Inheritance PatternMitochondrial Inheritance Pattern

Mitochondrial genes are passed from mothers to Mitochondrial genes are passed from mothers to offfspring.offfspring.

Only females pass on the genesOnly females pass on the genes

The 37 Mitochondrial GenesThe 37 Mitochondrial Genes

24 encode proteins important for protein 24 encode proteins important for protein synthesissynthesis Mutations can have devastating effectsMutations can have devastating effects

13 encode proteins needed for energy 13 encode proteins needed for energy productionproduction Mutations often affect skeletal muscle and Mutations often affect skeletal muscle and

cause fatiguecause fatigue

HeteroplasmyHeteroplasmy A mutation can occur in one mitochondrial DNA A mutation can occur in one mitochondrial DNA

ring and not another.ring and not another. When the mitochondria divide, different batches When the mitochondria divide, different batches

of daughter mitochondria are produced (some of daughter mitochondria are produced (some with the mutation, some without)with the mutation, some without)

It is therefore possible to have mutant It is therefore possible to have mutant mitochondrial DNA in some tissues but not mitochondrial DNA in some tissues but not othersothers

Causes variation is expressivity of a Causes variation is expressivity of a mitochondrial disease depending on which mitochondrial disease depending on which tissues/organs have cells with mutated tissues/organs have cells with mutated mitochondrial DNAmitochondrial DNA

LinkageLinkage

Two genes on the same chromosome may Two genes on the same chromosome may “co-segregate”“co-segregate”

Example: Dihybrid cross of pea plants with Example: Dihybrid cross of pea plants with purple flowers (Pp) and long pollen grains purple flowers (Pp) and long pollen grains (Ll)(Ll)

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross Self-cross

p

L l

P

L

p

l

Figure 5.10

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Self-cross

p

L l

P

L

p

l

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

Malegametes

PL

pl

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Phenotypic ratio 3:

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Phenotypic ratio 3:

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3:3

Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross

F1

Phenotypic ratio 3:1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3:3:1

Crossing Over May Disrupt LinkageCrossing Over May Disrupt Linkage

Linkage MapsLinkage Maps

The farther apart 2 The farther apart 2 genes are, the more genes are, the more likely their linkage will likely their linkage will be disrupted during be disrupted during crossing overcrossing over

% recombination tells % recombination tells us the relative us the relative location of the geneslocation of the genes